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<title>Unreliable Failure Detectors for Reliable Distributed Systems</title>
<!-- Changed by: Tushar Deepak Chandra, 16-Feb-1996 -->
<h1>Unreliable Failure Detectors for Reliable Distributed Systems</h1>
by
<!WA0><!WA0><!WA0><!WA0><!WA0><!WA0><!WA0><!WA0><a href="http://www.cs.cornell.edu/Info/People/chandra/home.html">Tushar Deepak Chandra</a> and
<!WA1><!WA1><!WA1><!WA1><!WA1><!WA1><!WA1><!WA1><a href="http://www.cs.cornell.edu/Info/People/sam/sam.html">
Sam Toueg.</a>
This paper has 51 pages.  To get a postscript copy of this paper, click
<!WA2><!WA2><!WA2><!WA2><!WA2><!WA2><!WA2><!WA2><a href="ftp://ftp.cs.cornell.edu/pub/chandra/failure.detectors.algorithms.ps.Z">here</a>
(or <!WA3><!WA3><!WA3><!WA3><!WA3><!WA3><!WA3><!WA3><a href="ftp://ftp.db.toronto.edu/pub/vassos/failure.detectors.algorithms.ps.Z">here</a>
to get it from the mirror site).  To appear in <!WA4><!WA4><!WA4><!WA4><!WA4><!WA4><!WA4><!WA4><a href="http://theory.lcs.mit.edu/~jacm/">JACM</a>.
<h2>Abstract</h2>
We introduce the concept of unreliable failure detectors
	and study how they can be used to solve Consensus
	in asynchronous systems	with crash failures.
We characterise unreliable failure detectors 
	in terms of two properties ---
	completeness and accuracy.
We show that Consensus can be solved even with unreliable failure detectors
	that make an infinite number of mistakes,
	and determine which ones
	can be used to solve Consensus
	despite any number of crashes,
	and which ones require a majority of correct processes.
We prove that Consensus and Atomic Broadcast are reducible to
	each other in asynchronous systems with crash failures;
	thus the above results also apply to Atomic Broadcast.
A companion paper shows that one of the failure detectors introduced here
	is the weakest failure detector for solving Consensus <!WA5><!WA5><!WA5><!WA5><!WA5><!WA5><!WA5><!WA5><a href="http://www.cs.cornell.edu/Info/People/chandra/WeakestFD.html">[CHT92]</a>.
<hr>
Research supported by an IBM graduate fellowship, NSF grants CCR-8901780 and
	CCR-9102231, DARPA/NASA Ames Grant NAG-2-593, and in part by Grants
	from IBM and Siemens Corp.
<hr>
<i>Maintained by <!WA6><!WA6><!WA6><!WA6><!WA6><!WA6><!WA6><!WA6><a href="http://www.cs.cornell.edu/Info/People/chandra/home.html">tushar@watson.ibm.com</a></i>
